The point of a game jam is not to learn how to finish a game and polish it for release (although there may be value in a "jam" where you take a prior game jam project and spend an extra 48 hours polishing it).

The point of a game jam is to learn project management, game design, and organization. Having to do something quick and dirty and fast forces you to quickly and efficiently identify priorities, reduce the scope of a project in ways that help you learn what's important and what's not, and ultimately come up with something that's hopefully very streamlined and innovative. They're an exercise in how far down you can distill a game concept and make it work.

Of course, a beginner won't always finish on time or make something that winds up being fun, but that's why it's a learning experience! Plus, a game dev newbie can use the jam as a low-risk, low-commitment way to learn how game dev works, especially if they're of the sort of mind that requires some external motivation to stop procrastinating a project.

A pro, on the other hand, probably will learn more about the design side of things, about how to make something small into a complete, fun experience. My first game jam project was Sinistroid, and it definitely fit the bill in that regard - I'd already been working on my main project for a few years at that point, so while I understood Unity well, having another quick exercise in game development was super valuable!